1:N protection in an optical terminal

ABSTRACT

A communication network comprising at least one first terminal, at least one second terminal, a plurality of links, and at least first and second nodes. The first node is bidirectionally coupled to the first terminal through at least a first one of the links, and also is bidirectionally coupled to the second terminal through at least a second link and the second node. Preferably, the first node comprises a plurality of communication paths, each of which is coupled at a first end thereof to at least one corresponding first link. Second ends of the communication paths are all coupled to the second link, through a multiplexing device, and route signals between the first and second links. The first node also preferably comprises at least one alternate communication path having a first end coupled through the multiplexing device to the second link, at least one switch that is coupled to the alternate communication path, and a detector for detecting a failure in at least one of the communication paths. A controller is coupled to the detector and the switch. The controller is responsive to the detector detecting a failure in at least one of the communication paths for controlling the switch to couple the alternate communication path to a corresponding first link, thereby enabling a signal to be routed between that first link and the second link through the alternate communication path.

FIELD OF THE INVENTION

This invention relates generally to optical communications networks,and, in particular, to an apparatus for providing 1:N (“one-to-N”)protection in an optical terminal of a Wavelength-Division Multiplexed(WDM) multi-channel optical communications network.

BACKGROUND OF THE INVENTION

It is known to provide protection in optical networks against linefailures, node failures, and the like, by equipping such networks withbypass equipment for bypassing failed components and routing signals totheir intended destinations. An example of a prior art network thatincludes bypass equipment is depicted in FIG. 1. The network includesoptical line terminals (OLTs), or nodes, 100 and 200, and a plurality ofterminals 100-1 to 100-n, 200-1 to 200-n. The nodes 100 and 200 arebidirectionally coupled to one another through a bidirectionaltransmission link (L).

The node 100 comprises a plurality of bidirectional communication pathsP-1 to P-n and P-1′ to P-n′ that are interposed between an interface(I′) and a WDM multiplexer/demultiplexer (MUX/DEMUX) 106 of the node100. Bidirectional transponders 102-1 to 102-n are included in thecommunication paths P-1 to P-n, respectively, and bidirectionaltransponders 104-1 to 104-n are included in the communication paths P-1′to P-n′, respectively, of node 100. Although not shown in FIG. 1, thenode 200 is assumed to include components which mirror those of node100.

Bidirectional links L100-1 to L100-n couple an interface IF1 of each ofthe terminals 100-1 to 100-n, respectively, to node interface (I′), andbidirectional links L200-1 to L200-n couple interface IF1 of each of theterminals 200-1 to 200-n, respectively, to the node 200. Similarly,bidirectional links BL100-1 to BL100-n couple an interface IF2 of eachof the terminals 100-i to 100-n, respectively, to the interface (I′) ofnode 100, and bidirectional links BL200-1 to BL200-n couple interfaceIF2 of the terminals 200-1 to 200-n, respectively, to the node 200.

Each of the terminals 100-1 to 100-n and 200-1 to 200-n normallytransceives signals through the interface IF1 of that terminal, andtransceives signals through the other interface IF2 only in cases wherethe interface IF1 and/or the link coupled thereto is inactive.Accordingly, the interface IF1 is known to persons skilled in the art asa “working” interface, and the links L100-1 to L100-n and L200-1 toL200-n coupled thereto are known as “working” links. Also, the interfaceIF2 is known in the art as a “protection” interface, the links BL100-1to BL100-n and BL200-1 to BL200-n are known as “protection” links, andthe transponders 104-1 to 104-n are known as “protection” transponders.Moreover, the terminals 100-1 to 100-n and 200-1 to 200-n are known as“protected” terminals, since they include the protection interface IF2,whereas terminals that do not include a protection interface IF2 areknown as “unprotected” terminals.

The so-called protected terminals operate in the following manner In theevent that a failure occurs in the interface IF1 of a terminal 100-1 to100-n, 200-1 to 200-n, and/or in a link or communication path coupled tothat interface, the terminal recognizes the occurrence of the failureand discontinues transceiving signals through the interface IF1.Assuming that the terminal also recognizes that the protection linkcoupled thereto is active, the terminal resumes transceiving the signalsover that protection link through the protection interface IF2. As aresult, the failed component is bypassed, and the signals arecommunicated through the various protection components of the network.

Unfortunately, the above-described network has drawbacks in that itrequires the use of many protection components (e.g., transponders 104-1to 104-n) in the nodes 100 and 200, and those nodes 100 and 200 aregenerally eypensive. Also, the above-described network does not provideany failure protection for unprotected terminals (not shown) that may beincluded in the network. Accordingly, it would be desirable to provide anetwork which overcomes the above-described drawbacks, and whichprovides protection against network component failures for bothprotected terminals and unprotected terminals. It would also bedesirable to provide an optical line terminal that is less expensivethan those of the prior art network described above.

SUMMARY OF THE INVENTION

It is a first object of this invention to provide a network whichprovides protection against network component failures for bothprotected and unprotected terminals

It is a another object of this invention to provide an improved opticalline terminal for a network, wherein the optical line terminal protectsagainst network component failures.

It is a further object of this invention to provide 1:N protection in anoptical line terminal that is less expensive than prior art lineterminals.

Further objects and advantages of this invention will become apparentfrom a consideration of the drawings and ensuing description.

The foregoing and other problems are overcome and the objects of theinvention are realized by a method for protecting against componentfailures in an optical communications network, and an opticalcommunications network that operates in accordance with the method. Inaccordance with one embodiment of the invention, the communicationsnetwork comprises at least one first terminal, at least one secondterminal, a plurality of links, and at least a first line node (alsoreferred to as an “optical line terminal”). The first line node isbidirectionally coupled to the first terminal through at least a firstone of the links, and is also bidirectionally coupled to a secondterminal through at least a second one of the links. The network ispreferably a Wavelength-Division Multiplexed (WDM) multi-channel opticalnetwork.

Preferably, the first line node comprises a plurality of firstcommunication paths, each of which has a first end and a second end. Thefirst end of each first communication path is coupled to a correspondingfirst link, and a second end of each first communication path is coupledthrough a multiplexing device to the second link. Each of the firstcommunication paths routes signals, received by the first line node,between a respective first link and the second link.

In accordance with an aspect of this invention, the first line node alsopreferably comprises (a) at least one first alternate communication pathhaving a first end coupled to the at least one second link through themultiplexing device, (b) at least one first switch that is coupled to asecond end of the first alternate communication path, and (c) a firstdetector. The first detector monitors the first communication paths foran occurrence of a failure in at least one of those paths. A firstcontroller is coupled to the first detector and the first switch. Thefirst controller is responsive to receiving information from the firstdetector indicating that a failure has been detected in at least one ofthe first communication paths for controlling the first switch to couplethe first alternate communication path to a corresponding one of thefirst links, thereby enabling a signal to be routed between that firstlink and the second link through the first alternate communication path.As a result, the failed communication path is bypassed and the signal isforwarded towards its intended destination.

In accordance with one embodiment of this invention, protection againstnetwork component failures is provided for “unprotected” terminals, andthe first line node is equipped with one or more splitters. Eachsplitter has an input and a first output that are both coupled in arespective one of the first communication paths. Each splitter splitssignals applied to its input terminal into corresponding signal portionsand outputs resulting first and second signal portions through the firstoutput and a second splitter output, respectively. In this embodiment,the first controller responds to receiving the information from thefirst detector by controlling the first switch to couple the secondoutput of the splitter from the failed path to the second link, throughthe alternate communication path.

Also in accordance with this embodiment of the invention, thecommunication network further comprises at least one second line nodethat is interposed between the second link and the second terminal. Thefirst and second line nodes are preferably coupled together through thesecond link, and the second line node is coupled to the second terminalthrough at least one third link. The second line node preferablycomprises a plurality of second communication paths, each of which has afirst end and a second end. The first ends of the second communicationpaths are coupled to the second link through a demultiplexing device,and the second end of each second communication path is coupled to acorresponding third link, for providing a communication route betweenthe second and third links. Each of the second communication paths iscoupled to a corresponding one of the first communication paths throughthe second link.

The second line node preferably also comprises (a) at least one secondalternate communication path having a first end that is coupled to thesecond link, (b) at least one second switch that is coupled to a secondend of the second alternate communication path, and (c) a seconddetector for detecting a failure in at least one of the secondcommunication paths. A second controller of the second line node ispreferably coupled to the second detector and the second switch. Thesecond controller is responsive to the second detector outputtinginformation indicating that a failure has been detected in at least oneof the second communication paths for controlling the second switch tocouple the second alternate communication path to a corresponding thirdlink, thereby enabling a signal to be routed between the second link andthat third link through the second alternate communication path.

Preferably, the first and second detectors detect failures in the firstand second communication paths, respectively, by detecting a loss oflight in those respective paths.

In accordance with another embodiment of the invention, at least one ofthe first and second controllers also responds to receiving theinformation from the first and second detector, respectively, bynotifying the other controller of the failure detected by that detector.The other controller then responds by implementing the above-describedswitching operation in its respective line node.

According to one embodiment of the invention, a transponder is includedin each of the first and second communication paths, and a transponderis included in each of the first and second alternate communicationpaths. The first and second controllers also respond to a detection of afailure by the first and second detector, respectively, by disabling thetransponder included in the failed path.

In accordance with still another embodiment of this invention, a linenode for providing 1:N protection for “protected” terminals is provided.In this embodiment the line node is coupled to 1) each of a plurality offirst terminals through both a first link and a second link, and 2) atleast one second terminal through at least one third link. Preferably,the line node comprises a plurality of communication paths for routingsignals being communicated between the first terminals and the at leastone second terminal. Each communication path has a first end coupled toa respective one of the first links and a second end coupled to the atleast one third link. The line node preferably also comprises at leastone switch having a plurality of first terminals and a second terminal.Each of the first terminals of the switch is coupled to a respective oneof the second links, and the second terminal of the switch is coupled tothe at least one third link.

A detector of the line node monitors for a failure in at least one ofthe line node communication paths, and provides an output to acontroller of the line node in response to detecting a failure in the atleast one path. The controller responds to receiving the detector outputby controlling the switch to couple a corresponding one of the secondlinks to the at least one third link, thereby providing an alternateroute through those links. Also, the protected first terminal which iscoupled to the failed communication path (through a corresponding firstlink) discontinues transceiving signals through its “working” interface,and resumes transceiving the signals through a “protection” interface ofthe terminal. As a result, communications between the protected firstterminal and the at least one second terminal resume through theestablished alternate route.

In accordance with a further embodiment of this invention, acommunications network is provided which includes both of the types ofline nodes described above, and which provides network failureprotection for both unprotected and protected terminals.

BRIEF DESCRIPTION OF THE DRAWINGS

The above set forth and other features of the invention are made moreapparent in the ensuing Detailed Description of the PreferredEmbodiments when read in conjunction with the attached drawings,wherein:

FIG. 1 shows a block diagram of an optical line terminal (OLT), or node,that is constructed in accordance with the prior art, and which isoptically coupled to a plurality of terminals;

FIG. 2, consisting of FIGS. 2 a and 2 b, shows an optical communicationsnetwork that includes terminals and nodes constructed and operated inaccordance with an embodiment of this invention;

FIG. 3 shows an optical communications network that includes terminalsand nodes constructed and operated in accordance with another embodimentof this invention;

FIGS. 4 a and 4 b are a logical flow diagram depicting a method inaccordance with one embodiment of this invention;

FIGS. 5 a and 5 b are a logical flow diagram depicting a method inaccordance with another embodiment of this invention; and

FIG. 6 shows an optical communications network that includes terminalsand nodes constructed and operated in accordance with a furtherembodiment of this invention.

Identical portions of the various figures have been identified with thesame reference numerals in order to simplify the description of thepresent invention. Components having similar purposes have beendesignated using the same reference numerals with a prime, double prime,or triple prime added.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 2 a and 2 b illustrate a block diagram of a plurality of nodes(also referred to as optical line terminals (OLTs)) 1 and 2 andterminals 10-1 to 10-n and 10-1′ to 10-n′ of an optical communicationsnetwork that is suitable for practicing this invention. The network isdepicted as a point-to-point communications network, although theinvention is not limited to being employed only in such networks. Forexample, the invention may also be implemented in a mesh orpoint-to-multipoint (chain) communications network.

The nodes 1 and 2 are coupled together via transmission links L1, L2,and L3, each of which may include, for example, one or more opticalfibers (e.g., two unidirectional fibers). In other embodiments of theinvention, two or all three of the communication links L1, L2, and L3may be combined into a single fiber link. The terminals 10-1 to 10-n arebidirectionally coupled to an interface I1 of the node 1 via “working”communication links L10-1 to L10-n, respectively, and the terminals10-1′ to 10-n′ are bidirectionally coupled to an interface I2 of thenode 2 via “working” communication links L10-1′ to L10-n′, respectively.

The terminals 10-1 to 10-n are assumed to have a capability forreceiving signals (e.g., optical signals) from and transmitting signalsto the node 1 by way of the links L10-1 to L10-n, respectively, and theterminals 10-1′ to 10-n′ are assumed to have a capability for receivingsignals (e.g., optical signals) from and transmitting signals to thenode 2 by way of the links L10-1′ to L10-n′, respectively. Each terminal10-1 to 10-n and 10-1′ to 10-n′ is also assumed to lack the capabilityand/or interface equipment for automatically switching to backup or“protection” links in the event of a failure in one or more of therespective links L10-1 to L10-n and L10-1′ to L10-n′, and/or in thenetwork communication paths coupled to those links. As was previouslydescribed, such terminals are known to those having skill in the art as“unprotected” terminals.

Each individual terminal 10-1- to 10-n and 10-1′ to 10-n′ may be, forexample, a node operating in accordance with the Asynchronous TransferMode (ATM) or the Internet Protocol (IP) (e.g., an ATM switch or IProuter), or a node of the Synchronous Optical Network (SONET). It shouldbe noted, however, that the present invention, broadly construed, is notlimited to any one particular type of communication protocol, standard,or network.

The node 1 preferably comprises a plurality of “working” communicationpaths CP1-CPn (channels) interposed between the interface I1 and amultiplexer 16 of the node 1, and another plurality of “working”communication paths CP1′-CPn′ interposed between the interface I1 and ademultiplexer 18 of the node 1. Preferably, splitters 12-1 to 12-n andtransponders (e.g., port cards) 14-1 to 14-n are included in thecommunication paths CP1-CPn, respectively. As shown in FIG. 2 a, aninput IP1 and an output OP1 of the individual splitters 14-1 to 14-n areboth coupled in the respective paths CP1-CPn. The communication pathsCP1′-CPn′ of FIG. 2 b preferably include transponders 15-1 to 15-n,respectively, and switches 22-1 to 22-n, respectively, although in otherembodiments the transponders 15-1 to 15-n need not be included in thosepaths CP1′-CPn′. Each switch 22-1 to 22-n has an input IP1″ that iscoupled at an end of a respective one of the paths CP1′-CPn′, and anoutput OP1″ that is coupled to a respective one of the terminals 10-1 to10-n through a respective link L10-1 to L10-n.

The node 2 also preferably comprises a plurality of “working”communication paths CP1″-CPn″ (channels) interposed between ademultiplexer 18′ and the interface I2 of the node 2 (FIG. 2 a), andanother plurality of “working” communication paths CP1′″-CPn′″interposed between a multiplexer 16′ and the interface 12 of the node 2(FIG. 2 b). The communication paths CP1″-CPn″ preferably includetransponders 14-1′ to 14-n′, respectively, and switches 22-1′ to 22-n′,respectively, although in other embodiments the transponders 14-1′ to14-n′ need not be included in those paths CP1″-CPn″. Each switch 22-1′to 22-n′ has an input IP1″ that is coupled at an end of a respective oneof the paths CP1″-CPn″, and an output OP1′″ that is coupled to arespective one of the terminals 10-1′ to 10-n′ through a respective linkL10-1′ to L10-n′. Preferably, splitters 12-1′ to 12-n′ and transponders(e.g., port cards) 15-1′ to 15-n′ are included in the communicationpaths CP1′″-CPn′″, respectively, wherein an input IP1′ and an outputOP1′ of the individual splitters 15-1′ to 15-n′ is coupled in therespective paths CP1-′″CPn′″, as shown in FIG. 2 b.

In accordance with an aspect of this invention, the nodes 1 and 2comprise protection modules 1 a and 2 a, respectively, that are employedto provide backup (i.e., “protection”) communication paths for routingsignals between the terminals 10-1 to 10-n and 10-1′ to 10-n′ in theevent that a failure occurs in a “working” communication path CP1-CPn,CP1′-CPn′, CP1″-CPn″, CP1-′″-CPn1″, as will be described further below.In accordance with a presently preferred embodiment of the invention,the protection module 1 a comprises the splitters 12-1 to 12-n, theswitches 22-1 to 22-n (FIG. 2 b), 1×N optical switches 13 and 25,“protection” transponders 17 and 19, a local controller 3, and a monitorblock 4. Similarly, the protection module 2 a preferably comprises thesplitters 12-1′ to 12-n′ (FIG. 2 b), the optical switches 22-1′ to22-n′, 1×N optical switches 13′ and 25′, “protection” transponders 17′and 19′, a local controller 3′, and a monitor block 4′.

Preferably, the switches 22-1 to 22-n and 22-1′ to 22-n′ are each 1×2optical switches, although in other embodiments, optical splitters maybe employed in lieu of those switches, and/or other suitable types ofmultiple position switches may be employed in place of two or more ofthe 1×2 switches. Also, in other embodiments the multiplexer 16 anddemultip lexer 18 of node 1 may be embodied as a singlemultiplexer/demultiplexer (MUX/DEMUX), and the multiplexer 16′ anddemultiplexer 18′ of node 2 also may be embodied as a single MUX/DEMUX,rather than as separate devices as depicted in FIGS. 2 a and 2 b, andeach transponder 14-1 to 14-n, 14-1′ to 14-n′, in combination with acorresponding one of the transponders 15-1 to 15-n, 15-1′ to 15-n′, mayrepresent a portion of a bidirectional transponder. Preferably, themultiplexers 16 and 16′ and demultiplexers 18 and 18′ areWavelength-Division Multiplex (WDM) devices.

The manner in which the various components of the nodes 1 and 2 operatewill now be described, beginning with those depicted in FIG. 2 a. Themonitor blocks 4 and 4′ monitor the communication paths CP1-CPn,CP1′-CPn′ and CP1″-CPn″, CP1′″-CPn′″, respectively, for the presence orabsence of light in those paths, and notify the respective controllers 3and 3′ of whether or not light has been detected in those paths. Adetection revealing that light is absent in a communication path isindicative of a failure in the path or a failure in a path coupledthereto. For example, light may be absent in the communication path as aresult of a failure of a corresponding transponder 14-1 to 14-n, 14-1′to 14-n′, 15-1 to 15-n, 15-1′ to 15-n′. Light also may be absent in acommunication path as a result of other failure-causing events, such as,for example, a transceiver and/or “working” interface failure in acorresponding terminal 10-1 to 10-n and 10-1′ to 10-n′, a failure of alink L10-1 to L10-n and L10-1′ to L10-n′, a failure of a node fiberdisconnect or optical amplifier (not shown), etc. The present embodimentof the invention protects primarily against failures occurring in 1)portions of the communication paths CP1-CPn interposed between thesplitters 12-1 to 12-n and multiplexer 16, 2) portions of thecommunication paths CP1′-CPn′ interposed between the switches 22-1 to22-n and demultiplexer 18, 3) portions of the communication pathsCP1″-CPn″ interposed between the demultiplexer 18′ and the switches22-1′ to 22-n′, and 4) portions of the communication paths CP1′″-CPn′″interposed between the multiplexer 16′ and the splitters 12-1′ to 12-n′,respectively.

The monitor blocks 4 and 4′ each may be embodied as one or more opticalsensors, such as a photodiode, although, for convenience, only the twomonitor blocks 4 and 4′ are shown in FIG. 2 a. In an exemplaryembodiment, each communication path CP1-CPn, CP1′-CPn′, CP1″-CPn″, andCP1′″-CPn1′″ may have its own dedicated optical sensor(s) for detectingthe presence or absence of light in the path. For example, the sensorsmay be integral parts of the transponders 14-1 to 14-n, 15-1 to 15-n,14-1′ to 14-n′, and 15-1′ to 15-n′ in the respective paths CP1-CPn,CP1′-CPn′, CP1″-CPn″, and CP1′″-CPn′″, or may be tapped into a selectedpoint in the paths adjacent to the transponders (although this also isnot shown for convenience).

The controllers 3 and 3′ function to coordinate the exchange of signalsbetween the nodes 1 and 2 and the exchange of signals between the nodes1 and 2 and the terminals 10-1 to 10-n and 10-1′ and 10-n′,respectively, in the event that a failure is detected in a communicationpath by a monitor block 4 or 4′. The controllers 3 and 3′ are coupled tothe switches 13 and 13′, respectively, and, although not shown in FIGS.2 a and 2 b, are also coupled to the switches 25, 22-1 to 22-n and 25′,22-1′ to 22-n′, respectively. The controllers 3 and 3′ control theconfigurations (i.e., positions) of those respective switches inresponse to receiving either a failure notification signal from monitorblock 4 or 4′, respectively, or a failure notification signal from theother controller. The controllers 3 and 3′ are bidirectionally coupledtogether through communication link L3, for communicating with oneanother (e.g., over an optical supervisory channel). The manner in whichthe controllers 3 and 3′ control the various switches 13, 13′, 25, 25′,22-1 to 22-n, and 22-1′ to 22-n′ to coordinate signal exchanges withinthe network in the event that a failure is detected will be describedbelow.

The splitters 12-1 to 12-n of node 1 are preferably passive splitters,and split signals received from the respective terminals 10-1 to 10-n(over respective links L10-1 to L10-n) into two corresponding signalportions, one of which is forwarded through splitter output OP1 to acorresponding transponder 14-1 to 14 n, and the other of which isforwarded through splitter output OP2 to a corresponding input of theswitch 13. In response to receiving a signal portion from acorresponding splitter 12-1 to 12-n, each transponder 14-1 to 14-noperates in a known manner for outputting to the multiplexer 16 acorresponding signal having a predetermined one of a plurality ofavailable wavelengths. Similarly, the transponder 17 responds toreceiving a signal output from the switch 13; by outputting acorresponding signal having a predetermined wavelength to themultiplexer 16.

As was previously described, the multiplexer 16 is preferably a WDMmultiplexer, and operates in a known manner for coupling differentwavelength signals received from the respective transponders 14-1- to14-n and 17 onto the transmission link L1 for transmission to the node2. Within the node 2, signals received from the transmission link L1 areapplied to the demultiplexer 18′, which, in turn, demultiplexes thereceived signals and outputs corresponding signals, each having apredetermined wavelength, to corresponding ones of the transponders14-1′ to 14-n′ and 17′. The transponders 14-1′ to 14-n′ and 17′ operatein a similar manner as the transponders 14-1 to 14-n and 17 of node 1described above, and each respond to receiving a signal from thedemultiplexer 18′ by outputting a signal having a correspondingpredetermined wavelength to the switches 22-1′ to 22-n′ and 13′,respectively.

Having described the various components of the nodes 1 and 2 depicted inFIG. 2 a, the components of those nodes shown in FIG. 2 b will now bedescribed. The node components depicted in FIG. 2 b are employed forforwarding communications originating from the terminals 10-1′ to 10-n′to corresponding ones of the terminals 10-1 to 10-n. Referring to bothFIGS. 2 a and 2 b, output links of the terminals 10-1′ to 10-n′ arecoupled (through connectors A-1′ to A-n′, respectively) to the inputIP1′ of respective ones of the splitters 12-1′ to 12-n′ (FIG. 2 b). Likethe splitters-12-1 to 12-n of node 1, the splitters 12-1′ to 12-n′ ofnode 2 are preferably passive splitters, and each split received signalsinto two corresponding portions, one of which is forwarded throughsplitter output OP1′ to a corresponding transponder 15-1′ to 15-n′, andthe other of which is forwarded through splitter output OP2′ to acorresponding input of the switch 25.

Each of the transponders 15-1′ to 15-n′ and 19′ is responsive toreceiving a signal for outputting a corresponding signal having apredetermined wavelength to the multiplexer 16′, which, in turn,multiplexes those output signals onto the transmission link L2 fortransmission to the node 1. The demultiplexer 18 of node 1 operates in asimilar manner as the demultiplexer 18′ described above, anddemultiplexes signals received from the transmission link L2 and outputssignals, each having a predetermined wavelength, to corresponding onesof the transponders 15-1 to 15-n and 19. Each transponder 15-1 to 15-nand 19 responds to receiving a respective one of those signals byoutputting a signal having a corresponding predetermined wavelength to acorresponding one of the switches-22-1 to 22-n and 25.

Referring to the flow diagram of FIGS. 4 a and 4 b, a method inaccordance with an embodiment of this invention will now be described.At block A1, the method is started, and it is assumed that the nodes 1and 2 are operating in a normal operating mode wherein all of the nodecommunication paths CP1-CPn, CP1′-CPn′, CP1″-CPn″, CP1′″-CPn′″ and arefunctioning properly. During this operating mode, light is detected oneach of those node communication paths by the monitor blocks 4 and 4′(‘y’ at block A2), and, as a result, the controller 3′ maintains theswitches 22-1′ to 22-n′ in a configuration for coupling outputs of therespective transponders 14-1′ to 14-n′ to the terminals 10-1′ to 10-n′,respectively, and the controller 3 maintains the switches 22-1 to 22-nin a configuration for coupling outputs of the respective transponders15-1 to 15-n to the terminals 10-1 to 10-n, respectively. It is alsoassumed that signals are being provided from terminal 10-1 to terminal10-1′ by way of the communication path CP1 of node 1, the transmissionlink L1, and the communication path CP1″ of node 2.

At some time later, it is assumed that the transponder 14-1′ ofcommunication path CP1″ fails, and that the monitor block 4′ detects thefailure in that path CP1″ (‘N’ at block A2). In response to detectingthe failure in the path CP1″, the monitor block 4′ notifies thecontroller 3′ that a failure has occurred in the path CP1″ (block A3).The controller 3′ then responds by 1) providing a failure signal to thecontroller 3 indicating that a failure has been detected in the pathCP1″, 2) configuring the switch 13′ to cause the switch 13′ to couplethe output of protection transponder 17′ to an input IP2′″ of switch22-1′, and 3) configuring the switch 22-1′ to couple that input IP2′″ toterminal 10-1′, via link L10-1′ (block A4).

The controller 3 responds to receiving the failure signal from thecontroller 3′ by correlating the failed communication path CP1″ to acorresponding “working” communication path (e.g., CP1) from node 1(block A5). For example, the controller 3 may perform this correlationoperation by correlating information (received from controller 3′)identifying the failed path CP1″ with corresponding, pre-storedinformation relating to corresponding path CP1 from node 1, although inother embodiments, other suitable correlation techniques may also beemployed. After block A5, the controller 3 configures the switch 13 tocause output OP2 of the splitter 12-1 from the path CP1 determined atblock A5, to an input of the transponder 17 (block A6).

As a result of the switching operations performed at blocks A4 and A6,the failed communication path CP1″ is bypassed, and a backupcommunication path is established which forwards signals originatingfrom terminal 10-1, to the terminal 10-1′ (block A7, FIG. 4 b). Theestablished backup communication path in this example includes thesplitter 12-1 (e.g., the splitter portion from input IP1 to output OP2),the switch 13, the transponder 17, and the multiplexer 16 of node 1, aswell as the transmission link L1, and the demultiplexer 18′, transponder17′, switch 13′, and switch 22-1′ of node 2. Preferably, the switchingoperations of blocks A4 and A5 are performed in a manner which minimizesthe amount of signal traffic lost as a result of the failure in thecommunication path CP1″.

At some time after the backup communication path is established, it isassumed that the communication path CP1″ which failed at block A2 isrepaired, and that, as a result, light is detected again in that path bythe monitor block 4′ (block A8). In response to detecting the presenceof the light in the path CP1″, the monitor block 4′ notifies thecontroller 3′ (block A9), which then responds by reconfiguring theswitch 22-1′ to cause the output of transponder 14-1′ to be coupledagain to the terminal 10-1′, via link L10-1′ (block A10). As a result,signals originating from the terminal 10-1 are routed again to theterminal 10-1′ by way of the communication path CP1 of node 1, thetransmission link L1, and the repaired communication path CP1″ of node 2(block A11). Control then passes back to Block A2 where the methodcontinues in the manner described above.

It should be appreciated in the view of the foregoing description thatthe switching configurations implemented in the nodes 1 and 2 in theabove example are also implemented in cases in which, for example, afailure is detected in the communication path CP1. As but one example,it is assumed that a failure occurs in the communication path CP1, butthe monitor block 4 does not detect a loss of light in the path CP1(owing to, e.g., the failure occurring at a point in the path CP1 afterthe monitored point and/or a failure in the block 4). It is also assumedthat the failure in communication path CP1 is detected as a loss oflight in the corresponding communication path CP1″ of node 2 by themonitor block 4′. In this case, the controller 3′ responds to thefailure detection by configuring the switches 13′ and 22-1′ of node 2 inthe above-described manner (see, e.g., block A4), and by notifying thecontroller 3 of node 1 of the detected failure. The controller 3responds to receiving the notification by the performing operations ofblocks A5 and A6 described above to configure the switch 13 in theabove-described manner for establishing the alternate communicationpath.

As also can be appreciated in the view of the foregoing description, inthe event that a failure is detected in another other one of thecommunication paths CP1-CPn, CP1′-CPn′, CP1″-CPn″, and CP1′″-CPn′″,another suitable switching configuration is implemented by thecontrollers 3 and 3′ for bypassing the failed path. For example, in acase where a failure is detected in communication path CPn′ depicted inFIG. 2 b, the switches 25′, 25, and 22-n′ are configured to provide analternate communication path for routing signals received from terminal10-n′, to terminal 10-n.

It should be noted that although the foregoing method of the inventionis described in the context of one controller 3 or 3′ notifying theother controller when a failure is detected in a communication path,each individual controller 3 and 3′ may also make such a determinationindependently. For example, a failure in communication path CP1 of node1 may be detected as a loss of light in both that path (by the monitorblock 4) and corresponding communication path CP1″ of node 2 (by monitorblock 4′), and the appropriate switching operations may be implementedin those nodes separately to establish the backup communication path. Itshould further be noted that although the above-described embodiment hasbeen described in the context of providing protection for unprotectedterminals, it is also within the scope of this invention to employ thatembodiment for providing protection for protected terminals as well. Forexample, the components of modules 1 a and 2 a may be included in backupcommunication paths (described below) of nodes coupled to unprotectedterminals, for providing redundant backup protection.

Another aspect of the invention will now be described. In accordancewith this aspect of the invention, failure protection is provided withinnodes of a communication network for so called “protected” terminals. Aswas described above, “protected” terminals are known to those skilled inthe art as being equipped with “protection” interface equipment.

Referring to FIG. 3, a block diagram is shown of nodes 1′ and 2′ and“protected” terminals 10-1″ to 10-n″ and 10-1′″ to 10-n′″ of an opticalcommunications network that is suitable for practicing this aspect ofthe invention. The nodes 1′ and 2′ are bidirectionally coupled to oneanother through a bidirectional transmission link L4. Bidirectional“working” communication links L10-1″ to L10-n″ couple “working”interfaces (IF1) of the terminals 10-1″ to 10-n″, respectively, to aninterface I1′of the node 1′, and bidirectional “protection”communication links BL1-BLn couple “protection” interfaces (IF2) ofthose terminals 10-1″ to 10-n″, respectively, to the interface I1′.Similarly, bidirectional “working” communication links L10-1′″ toL10-n′″ couple “working” interfaces (IF1) of the terminals 10-1′″ to10-n′″, respectively, to an interface I2′ of the node 2′, andbidirectional “protection” communication links BL1′-BLn′ couple“protection” interfaces (IF2) of the terminals 10-1′″ to 10-n′″,respectively, to the interface I2′.

The node 1′ comprises a plurality of “working” bidirectionalcommunication paths P1-Pn that are interposed between interface I1′ anda multiplexer/demultiplexer (MUX/DEMUX) 34 of the node 1′, and the node2′ comprises a plurality of bidirectional “working” communication pathsP1′-Pn′ that are interposed between a multiplexer/demultiplexer(MUX/DEMUX) 34′ of node 2′ and interface I2′ of the node 2′.Bidirectional transponders 30-1 to 30-n are included in thecommunication paths P1-Pn, respectively, of node 1′, and bidirectionaltransponders 30-1′ to 30-n′ are included in the communication pathsP1′-Pn′, respectively, of the node 2′.

In accordance with an aspect of this invention, the nodes 1′ and 2′ alsocomprises protection modules 1 a′ and 2 a′, respectively. The protectionmodule 1 a′ preferably comprises a monitor block 4″, a controller 3″, abidirectional 1×N switch 31 having terminals T1-Tn that are coupledthrough interface I1′ to respective ones of the protection communicationlinks BL1-BLh, and a bidirectional “protection” transponder 32 that isinterposed between another terminal T′ of the switch 31 and theMUX/DEMUX 34. Similarly, the protection module 2 a′ preferably comprisesa monitor block 4′″, a controller 3′″, a bidirectional 1×N switch 31′having terminals T1′-Tn′ that are coupled through the interface I2′ torespective ones of the protection communication links BL1′-BLn′, and abidirectional “protection” transponder 32′ that is interposed betweenanother terminal T″ of the switch 31′ and the MUX/DEMUX 34′. Thecontrollers 3″ and 3′″ are bidirectionally coupled to one another by wayof a communication link L5, although in other embodiments, thecontrollers 3″ and 3′″ may communicate with one another through the linkL4.

The controllers 3″ and 3′″ and monitor blocks 4″ and 4′″ are similar tothose described above, and will not be described in further detail. Thetransponders 30-1 to 30-n, 32, 30-1′ to 30-n′, and 32′ each areresponsive to receiving a signal for outputting a corresponding signalhaving a predetermined wavelength.

The MUX/DEMUX 34 is preferably a WDM device, and operates in a knownmanner for coupling signals having respective wavelengths from therespective transponders 30-1 to 30-n and 32 onto the transmission linkL4 for transmission to the node 2′. The MUX/DEMUX 34 also demultiplexessignals received from the link L4 and outputs signals, each having apredetermined wavelength, to corresponding ones of the transponders 30-1to 30-n and 32. Preferably, the MUX/DEMUX 34′ also is a WDM device, andoperates by coupling signals having respective frequencies, receivedfrom the respective transponders 30-1′ to 30-n′ and 32′, onto thetransmission link L4 for transmission to the node 1′, and bydemultiplexing signals received from the link L4 for outputtingcorresponding signals, each having a predetermined wavelength, tocorresponding ones of the transponders 30-1′ to 30-n′ and 32′.

The switches 31 and 31′ are controllable by the controllers 31″ and 3′″,respectively, for being placed in a particular configuration forproviding a backup communication path in the event that a communicationpath failure is detected by monitor block 4″ and/or 4′″, respectively,as will be further described below. It should be noted that, althoughfor simplicity the various components L10-1″ to L10-n″, L10-1′″ toL10-1′″, BL1-BLn, BL1′-BLn′, P1-Pn, P1′-Pn′, 31, 31′, 32, 32′, 30-1 to30-n, 30-1′ to 30-n′, L4, and L5 of FIG. 3 are described herein in thecontext of being bidirectional, in other, preferred embodiments,corresponding unidirectional components may be employed instead. Forexample, two or more unidirectional links may be employed for each linkL4, L5, L10-1″ to L10-n″, L10-1′″ to L10-1′″, BL1-BLn, and BL1′-BLn′,two or more unidirectional transponders may be employed in lieu of eachtransponder 31, 31′, 32, 32′, 30-1 to 30-n, 30-1′ to 30-n′, each pathP1-Pn, P1′-Pn′ may include two or more unidirectional paths, two or more1×N switches may be employed in lieu of each switch 31 and 31′, and aseparate multiplexer and demultiplexer may be employed in lieu of eachMUX/DEMUX 34, 34′. For these embodiments, the manner in which suchunidirectional components would be interconnected within the overallsystem would be readily appreciated by one skilled in the art, in viewof this description.

Referring to the flow diagram of FIGS. 5 a and 5 b, a method inaccordance with this aspect of the invention will now be described. Atblock A1′, the method is started, and it is assumed that the terminals10-1″ to 10-n″ and 10-1′″ to 10-n′″ are employing their “working”interfaces for communicating, through nodes 1′ and 2′, link L4, and thelinks L10-1″ to L10-n″ and L10-1′″ to L10-n′″, respectively.

At block A2′ it is assumed that one of the monitor blocks 4 or 4′detects a loss of light in one of the paths P1-Pn or P1′-Pn′,respectively (‘N’ at block A2′). For example, the loss of light in thepath may be a result of a failure of a corresponding transponder 30-1 to30-n or 30-1′ to 30-n′, a failure of a “working” transceiver and/orinterface (IF1) of a corresponding transmitting terminal, a failure of a“working” link coupled to a transmitting terminal, respectively, and/ora failure of a node fiber disconnect or optical amplifier (not shown) inthe path, etc. For the purposes of this description, it is assumed thatthe “working” interface (IF1) of terminal 10-1″ fails and that, as aresult, the monitor block 4″ detects the absence of light in thecommunication path P1.

In response to detecting the absence of light in the path P1, themonitor block 4″ notifies the controller 3″ that a failure has occurredin the path P1 (block A3′). The controller 3″ then responds by 1)providing a failure signal to the controller 3′″ to notify thecontroller 3′″ of the failure in the path P1, and 2) configuring theswitch 31 to cause the switch 31 to couple the protection link BL1 tothe “protection” transponder 32, through interface I1′ (block A4′). Thecontroller 3′″ responds to receiving the failure signal from thecontroller 3″ by correlating the failed communication path P1 tocorresponding “working” communication path P1′ of node 2, in the mannerdescribed above (block A5′), and by thereafter configuring the switch31′ to cause the output of transponder 32′ to be coupled to theprotection link BL1′, through interface I2′ and switch 31′ (block A6′).

As a result of the switching operations performed at blocks A4′ and A6′,an alternate (backup) communication path is established between theterminals 10-1″ and 10-1′″ for bypassing the failed “working” interface(IF1) of terminal 10-1″ (block A7′). The alternate communication pathincludes, in this example, link BL1, switch 31, transponder 32,MUX/DEMUX 34, link L4, MUX/DEMUX 34′, transponder 32′, switch 31′, andthe link BL1′.

At block A8′, it is assumed that the terminals 10-1″ and ˜10-1′″ eachrecognize that the alternate communication path has been established(i.e., is active), and respond by switching to their protectioninterfaces (IF2) for resuming communications with one another throughthe alternate communication path established at block A7′. For example,the terminals 10-1″ and 10-1′″ may recognize that the alternatecommunication path has been established in response to detecting lightand/or a communication signal received from the (now active) protectionlinks BL1 and BL1′, respectively, or by some other known technique.

At some later time, it is assumed that 1) the failure which occurred atblock A2′ is repaired (e.g., the “working” interface (IF1) of terminal10-1″ is repaired and becomes operable again) (block A9′), and 2) thisis recognized by the terminals 10-1″ and 10-1′″, using a known techniqueas described above (e.g., the terminals 10-1″ and 10-1′″ may detectlight and/or a communication signal received from the protection linksBL1 and BL1′). The terminals 10-1″ and 10-1′″ then respond in a knownmanner by switching to their “working” interfaces (IF1), for resumingcommunications with one another through those interfaces (IF1),“working” links L10-1″ and L10-1′″, communication path P1, andcommunication path P1′ (block A10′). Control then passes back to blockA2′ where the method continues in the manner described above.

In view of the foregoing description, it can be appreciated that thenodes 1′ and 2′ provide 1:N protection for the protected terminalscoupled to those nodes, using a lesser number of components (e.g.,transponders and links) than are employed in the prior art nodes 100 and200 described above.

A further aspect of the invention will now be described, with referencebeing made to FIG. 6, which is a block diagram of nodes 1″ and 2″,unprotected terminals 10-1 and 10-1′, and protected terminals 10-1″ and10-1′″, of an optical communications network that is suitable forpracticing this aspect of the invention. In accordance with this aspectof the invention, the nodes 1″ and 2″ have a capability for providingbackup protection for both the protected and unprotected terminals.

The network includes similar components 10-1, 10-1′, 10-1″, and 10-1′″,12-1, 12-1′, 30-1, 30-1′, 30-1″, 30-1′″, 32, 32′, 31, 31′, 22-1, 22-1′,34, 34′, L4, L5, I1′, I2′-311, 3′″, 4″, and 4′″, as those describedabove, and thus those components will not be described in furtherdetail. In accordance with a preferred embodiment of the invention, thenodes 1″ and 2″ also comprise switches 31 a and 31 a′, respectively.Preferably, a first input T1 a of the switch 31 a is coupled to anoutput OP2 of splitter 12-1, a second input T2 a of the switch 31 a iscoupled to an output of terminal 10-1″ through the interface I1′ and a“protection” link BL-1″, and an output Ta′ of the switch-31 a is coupledto an input of “protection” transponder 32. Similarly, a first input T1a′ of switch 31 a′ is preferably coupled to an output OP2′ of splitter12-1′, a second input T2 a′ of the switch 31 a′ is preferably coupled toan output protection interface IF2 of terminal 10-1′″ through theinterface I2′ and “protection” link BL-1′″, and an output Ta″ of theswitch 31 a′ is preferably coupled to an input of “protection”transponder 32′.

Also in the preferred embodiment of the invention, an input T′ of switch31 is coupled to an output of the transponder 32, a first output T′ ofthe switch 31 is coupled to a first input IP1″ of switch 22-1, and asecond output T2′ of the switch 31 is coupled to a protection interfaceIF2 of terminal 10-1″ through the node interface I1′ and protection linkBL-1 a″. Similarly, an input T″ of switch 31′ of node 2″ is coupled toan output of transponder 32′, a first output T1″ of the switch 31′ iscoupled to a first input IP1′″ of switch 22-1′, and a second output T2′″of the switch 31′ is coupled to a protection interface IF2 of terminal10-1′″ through the interface I2′ and a protection link BL-1 a′″. Also,an output OP1″ of the switch 22-1 of node 1″ is coupled to an input ofterminal 10-1 through the interface I1′ and a link L10-1 b, a secondinput IP2″ of switch 22-1 is coupled to an output of the transponder30-1, and, as was previously described, the first input IP1″ of theswitch 22-1 is coupled to the first output T1′ of switch 31. Likewise,an output OP1′″ of switch 22-1′ of node 2″ is coupled to an input ofterminal 10-1′ through the interface I2′ and a link L10-1 b′, a secondinput IP2′″ of switch 22-1′ is coupled to an output of the transponder30-1′, and, as was previously described, the first input IP1′″ of theswitch 22-1′ is coupled to the first output T1″ of switch 31′. Alsoshown in FIG. 6 is a link L10-1 a, which couples an output of terminal10-1 to splitter 12-1 through interface I1′, a link L10-1″, whichbidirectionally couples terminal 10-1″ to transponder 30-1″ throughinterface I1′ a link L10-1 a′, which couples an output of terminal 10-1′to splitter 12-1′ through interface I2′, and a link L10-1′″, whichbidirectionally couples terminal 10-1′″ to transponder 30-1′″ throughinterface I2′.

The manner in which the components of nodes 1″ and 2″ operate inresponse to a detection of a failure in the paths CP1 and/or CP1″ issimilar to that described above and shown in FIGS. 4 a and 4 b. However,in this embodiment, the controller 3′″ responds to receiving anotification from the monitor block 4′″ indicating that a failure hasbeen detected in path CP1″ (block A3) by 1) providing a failure signalto the controller 3″ indicating that a failure has been detected in thepath CP1″, 2) configuring the switch 31′ to cause that switch 31′ tocouple the output of protection transponder 32′ to the first input-IP1′″of switch 22-1′, and 3) configuring the switch 22-1′ to couple its firstinput IP1′″ to terminal 10-1′, via link L10-1 b′ (block A4). Also, atblock A6 the controller 3″ configures the switch 31 a to couple outputOP2 of the splitter 12-1 from path CP1, to the transponder 32.

As a result of these switching operations, the failed communication pathCP1″ is bypassed, and an alternate communication path is established forrouting signals output from the terminal 10-1 towards the terminal 10-1′(block A7). The established alternate communication path in this exampleincludes the splitter 12-1, the switch 31 a, the transponder 32, and theMUX/DEMUX 34 of node 1″, as well as the transmission link L4, and theMUX/DEMUX 34′, transponder 32′, switch 31′, and switch 22-1′ of node 2″.Also, at block A10, the controller 3′″ responds to receiving thenotification from monitor block 4′″ at block A9 by reconfiguring theswitch 22-1′ to cause an output of the transponder 30-1′ to be coupledagain to the terminal 10-1′, by way of link L10-1 b′.

The manner in which the components of nodes 1″ and 2″ operate inresponse to a detection of a failure in the paths P1 and/or P1″ issimilar to that shown and described above with reference to FIGS. 5 aand 5 b. However, in this embodiment, the controller 3″ responds toreceiving the failure notification at block A3′ by 1) providing, afailure signal to the controller 3′″ to notify the controller 3′″ of thefailure in the path P1, and 2) configuring the switch 31 a to couple theterminal 10-1″ to the “protection” transponder 32′, through protectionlink BL-1″ and interface I1′ (block A4′). Also, at block A6′ thecontroller 3′″ of node 2″ configures the switch 31′ to couple thetransponder 32′ to the terminal 10-1′″, through interface I2′ andprotection link BL-1 a′″.

As a result of these switching operations, an alternate communicationpath is established between the terminals 10-1″ and 10-1′″ (block A7′),and includes, in this example, link BL-1″, switch 31 a, transponder 32,MUX/DEMUX 34, link L4, MUX/DEMUX 34′, transponder 32′, switch 31′, andthe link BL-1 a′″.

It should be noted that although this embodiment of the invention isdescribed in the context of there being only the four terminals 10-1,10-1′, 10-1″, and 10-1′″ included in the network, more or less than thisnumber of terminals may also be provided, and, as one skilled in the artwould appreciate in view of this description, the switching arrangementsdepicted in FIG. 6 may be modified as deemed suitable to accommodatethat number of terminals. It should also be noted that, although forsimplicity the various components L10-1″, L10-1′″, 30-1, 30-1′, 30-1″,30-1′″, L4, L5, 32, 32′, P1, and P1″ of FIG. 6 are described herein inthe context of being bidirectional, in other, preferred embodiments,corresponding unidirectional components may instead be employed. Forexample, two or more unidirectional links may be employed for each linkL4, L5, L10-1″, and L10′″, two or more unidirectional transponders maybe employed in lieu of each transponder 30-1, 30-1′, 30-1″, 30-1′″, 32,and 32′, each path P1 and P1″ may include two or more unidirectionalpaths, and a separate multiplexer and demultiplexer may be employed inlieu of each MUX/DEMUX 34, 34′. Those having skill in the art wouldreadily appreciate, in view of this description, the manner in whichthose components would be interconnected within the overallcommunication system.

A further embodiment of this invention will now be described, withreference again being made to FIG. 6. In accordance with thisembodiment, the element 22-1 includes a 1×N coupler rather than aswitch, and couples signals received at each of the inputs IP1″ and IP2″to the output OP1″. Also in this embodiment, the element 22-1′ alsoincludes a 1×N coupler rather than a switch, and couples signalsreceived at each of the inputs IP1′″ and IP2′″ to the output OP1′″. Themanner in which the remaining components of the communication systemoperate is similar to that described above, except that no control ofswitches 22-1, 22-1′ is performed, and the switching operations involvedisabling (de-activating) and/or enabling (activating) selected ones ofthe transponders, as will be described below.

In accordance with this embodiment of the invention, the transponders30-1, 30-1″, and 32 are controllable by the controller 3″ for beingeither activated or de-activated (e.g., turned on or off), and thetransponders 30-1′, 30-1′″, and 32′ of node 2″ are controllable by thecontroller 3′″ for being either activated or de-activated. For example,the controller 3′″ responds to receiving the notification from themonitor block 4′″ at block A3 by 1) providing the failure signal to thecontroller 3″, 2) providing an enable signal to the transponder 32′ toenable that transponder, if it is not already enabled, and 3) providinga control signal to the transponder 30-1′ of the failed path CP1″ forcausing that transponder to become disabled 30-1′ (block A4). Also, atblock A6, in addition to configuring the switch 31 a in theabove-described manner, the controller 3″ 1) enables the transponder 32,if not already enabled, and 2) disables the transponder 30-1 of node 1″.As a result of these operations, signals that may be traversing the pathCP1, the link L4, and/or the portion of the path CP1″ appearing beforethe transponder 30-1′, are prevented from reaching the terminal 10-1′(e.g., the failed communication path CP1″ is bypassed), and thealternate communication path is established for routing signals receivedfrom terminal 10-1, to terminal 10-1′.

Also, at block A10, the controller 3′″ responds to the notificationreceived at block A9 by 1) notifying the controller 3″ that the pathCP1″ has been repaired, 2) enabling the transponder 30-1′, and 3)disabling the transponder 32′. Similarly, the controller 3″ of node 1″responds to receiving the notification from the controller 3′″ by 1)enabling the transponder 36-1, and 2) disabling the transponder 32. As aresult, the communication paths CP1 and CP1″ become active again forrouting signals received from terminal 10-1, to the terminal 10-1′, andany signals that may be traversing the portion of the alternatecommunication path appearing before the transponder 32′ are preventedfrom reaching the terminal 10-1′.

It should be noted that it is not necessary to disable the transpondersfrom both of the nodes 1″ and 2″ in order to prevent signals fromreaching the terminal 10-1′. For example, in the case described above,only the transponder 30-1′ or 32′ of the node 2″ closest to a receivingterminal 10-1′ need be disabled to prevent signals applied to thosedevices from reaching that terminal 10-1′. It should also be noted thatthe transponders included in the nodes of the previously describedembodiments (including, e.g., the one shown in FIG. 3) may also becontrolled in the above-described manner for being enabled/disabled, and1×N coupling devices may be employed in lieu of respective ones of theswitches included in those embodiments.

Although the invention has been described above in the context of thevarious switching operations being implemented in response to adetection of a failure in a single communication path within a node, itis also within the scope of this invention to implement those operationsin response to a detection of a failure in two or more of those paths.For example, and referring to the embodiment shown in FIG. 3, if monitorblock 4″ detects a failure in two or more of the paths P1-Pn of node 1′,the controller 3″ may respond by controlling the switch 31 to couple aselected, predetermined one of the switch inputs T1-Tn to the switchoutput T′, for bypassing the failure in a corresponding one of thepaths. Which one of the switch inputs T1-Tn is selected may bepre-programmed into the controller 3″, and may be predetermined inaccordance with applicable design and/or system operating criteria.

Moreover, although the invention has been described above in the contextof the controllers 3, 3′, 3″, and 3′″ being located within theprotection modules 1 a, 2 a, 1 a′, and 2 a′, respectively, the inventionis not limited to only such a configuration. By example, in otherembodiments the controllers 3, 3′, 3″, 3′″ may be located in otherportions of the respective nodes. Also, it should be noted that althoughthe invention is described in the context of the various switches beingconfigured in response to a detection made by a monitor block 4, 4′, 4″,4′″, those switches may be configured in response to other suitabletriggering events. As an example, it is within the scope of thisinvention to configure the switches in response to a user enteringconfiguration command information into one or more of the controllers 3,3′, 3″ and 3′″, using a user interface. It is also within the scope ofthis invention to employ the optical sensors outside of one or more ofthe nodes 1, 1′, 2, and 2′ for detecting failures in, for example, thevarious links L10-1 to L10-n, L10-1′ to L10-n′, L10-1″ to L10-n″, andL10-1′″ to L10-n′″, or to include sensor(s) in only one of the nodes.

While the invention has been particularly shown and described withrespect to preferred embodiments thereof, it will be understood by thoseskilled in the art that changes in form and details may be made thereinwithout departing from the scope and spirit of the invention.

1.-68. (canceled)
 69. A communication network, comprising: at least one first terminal; at least one second terminal; a plurality of links; and at least one first node, bidirectionally coupled to both the at least one first terminal through at least a first one of the links and to the at least one second terminal through at least a second one of the links, the at least one first node also being coupled to the at least one first terminal through an additional link, the at least one first node comprising: a plurality of first communication paths, each of the first communication paths being coupled at a first end thereof to at least one corresponding first link, wherein second ends of the first communication path are all coupled to the at least one second link, for providing a communication route between the first and second links, at least one first alternate communication path having a first end coupled to the at least one second link and a second end coupled to the additional link, at least one first switch coupled to the at least one first alternate communication path and coupled to the additional link through the at least one first alternate communication path, a first detector for detecting a failure in at least one of the plurality of first communication paths, and a first controller coupled to the first detector and to the at least one first switch, the first controller being responsive to the first detector detecting a failure in at least one of the first communication paths for controlling the at least one first switch to couple the at least one first alternate communication path and the additional link to the second link for routing a signal between the at least one first and second terminals through the at least one first alternate communication path and the additional link.
 70. A communication network as set forth in claim 69, further comprising at least one second node interposed between the at least one second link and the at least one second terminal, the at least one first and second nodes being coupled together through the at least one second link, the at least one second node being coupled to the at least one second terminal through at least one third link, and wherein the at least one second node comprises: a plurality of second communication paths, each having a first end and a second end, the first ends of the second communication paths being coupled to the at least one second link, the second end of each second communication path being coupled to a corresponding third link, for providing a communication route between the second and third links, at least one second alternate communication path having a first end coupled to the at least one second link, at least one second switch coupled to the at least one second alternate communication path, a second detector for detecting a failure in at least one of the plurality of second communication paths, and a second controller coupled to the second detector and to the at least one second switch, the second controller being responsive to the second detector detecting a failure in at least one of the second communication paths for controlling the at least one second switch to couple the at least one second alternate communication path to a corresponding third link, for routing a signal between that at least one second link and the third link through the at least one second alternate communication path.
 71. A communication network as set forth in claim 70, wherein the first and second detectors detect the failure in the first and second communication paths, respectively, by detecting a loss of light in those respective paths.
 72. A communication network as set forth in claim 70, wherein the at least one first node further comprises at least one first multiplexer/demultiplexer, the at least one second node further comprises at least one second multiplexer/demultiplexer, and wherein each of the first communication paths is coupled to a respective one of the second communication paths through the at least one second link and the first and second multiplexer/demultiplexer.
 73. A communication network as set forth in claim 69, further comprising at least one splitter, wherein each splitter has an input terminal and a first output terminal that are both coupled in a respective one of the at least one first communication paths, and the first controller responds to the first detector detecting a failure in a first communication path by controlling the at least one first switch to couple a second output terminal of a corresponding splitter to the at least one second link through the at least one alternate communication path.
 74. A communication network as set forth in claim 70, wherein the second switch has an input terminal and a plurality of output terminals, the input terminal of the second switch being coupled in the at least one second alternate communication path, and wherein the second node further comprises a plurality of third switches, each third switch having a first input terminal coupled in a corresponding one of the second communication paths, a second input terminal coupled to a corresponding one of the output terminals of the second switch, and an output terminal coupled to the at least one third link, and wherein the second controller responds to the second detector detecting a failure in a second communication path by controlling the second switch to couple signals received over the at least one second link to the second input terminal of the third switch coupled in the path, and by controlling that third switch to further couple those signals to the at least one third link.
 75. A communication network as set forth in claim 73, wherein the first node further comprises a multiplexer having an output coupled to the at least one second link, a first input coupled to an output of the first switch, and a plurality of second inputs each of which is coupled to a second end of a corresponding one of the first communication paths.
 76. A communication network as set forth in claim 73, wherein the first node comprises a plurality of transponders, individual ones of the transponders being interposed in respective ones of the first communication paths.
 77. A communication network as set forth in claim 76, and further comprising another transponder interposed in the at least one first alternate communication path.
 78. A communication network as set forth in claim 70, wherein the first and second controllers are coupled together through the at least one second link, the second controller also is responsive to the second detector detecting the failure in the second communication path for notifying the first controller of the failure, and wherein the first controller responds thereto by coupling the at least one first alternate communication path to a corresponding first link.
 79. A communication network as set forth in claim 73, wherein each first terminal transmits signals over only those ones of the first and second links that are coupled to the terminal and determined to be active by that terminal.
 80. A communication network as set forth in claim 69, wherein the first detector detects the failure in the at least one first communication path by detecting a loss of light in that path.
 81. A method for operating at least one line node of a communication network, the line node having a plurality of communication paths, each of which is coupled at a first end thereof through a first link to a first interface of a respective one of a plurality of first terminals, each communication path having a second end coupled through at least one second link to at least one second terminal, the line node also being coupled to a second interface of each first terminal through at least one third link, the method comprising: monitoring for a failure in at least one of the communication paths; and in response to detecting a failure in at least one of the communication paths, switchably coupling a corresponding at least one of the third links to the at least one second link.
 82. A method as set forth in claim 81, further comprising: detecting the failure in the at least one communication path at the first terminal coupled to that path; and in response to detecting the failure at the first terminal, providing a signal from the second interface of the first terminal to the line node through the third link coupled to the first terminal.
 83. A method as set forth in claim 81, further comprising notifying another node in the communication network of the detected failure.
 84. A method as set forth in claim 81, further comprising prior to the monitoring: providing a switch having a first terminal and a plurality of second terminals in the line node; connecting the first terminal of the switch to the at least one second link; connecting each second terminal of the switch to a respective one of the third links; and wherein the switchably coupling includes operating the switch to couple the corresponding third link to the at least one second link.
 85. A method as set forth in claim 81, wherein the monitoring is performed in at least one other line node of the network.
 86. A method as set forth in claim 85, further comprising notifying the line node of the failure in response to the other line node detecting a failure in the at least one communication path, and wherein the switchably coupling is performed in response to the notifying.
 87. A method as set forth in claim 81, wherein a transponder is included in each of the communication paths, and wherein in response to the failure being detected in the at least one communication path, the transponder included in that path is disabled.
 88. A method as set forth in claim 87, further comprising: detecting when the failed communication path becomes active again; and in response to detecting that the failed communication path has become active again, enabling the transponder included in that path.
 89. A method as set forth in claim 87, further comprising prior to the monitoring: providing at least one coupler having a first terminal and a plurality of second terminals in the line node; coupling the first terminal of the at least one coupler to the at least one second link; and coupling each second terminal of the at least one coupler to a respective one of the third links; and coupling a further transponder between the first terminal of the at least one coupler and the at least one second link, and wherein the switchably coupling is performed by enabling the further transponder.
 90. A method as set forth in claim 81, wherein the monitoring includes monitoring for a loss of light in the at least one communication path. 